In prior art it is known the advantage of adjusting automatically during the rolling process the position of the rolling cylinders to adapt progressively the pass thickness to the needs of progressive reduction of the thickness of the rolled section.
It is known that the rolling products, during their rolling, have a temperature variableness in their length and therefore a yielding variableness (compression strength or pressing strength) which determines different loads on the cylinders, that is different yielding of the structure.
Thus if the tail of a rod is colder than the head, during the rolling, the thickness of the rolled section tends to be higher on the tail because of a slight yielding of the cylinders which under a higher stress are more spaced (presence of unavoidable slacks and higher bending stress), or vice-versa in warmer areas of the rolled section.
The others presence of different temperatures (e.g., between head and tail) causes the problem of lack of size homogeneity in the traditional stands of obtaining products because of the different rolling loads deriving from the uneven temperatures.
This results in the difficulty rolled products with lower tolerances with the traditional rolling stands, particularly for the rolling of long products.
The servoassisted adjustment of the rolling cylinders is traditionally adopted for the rolling of large products, that is sheet plates, bands and large flats. This is carried out by using a quarter-stand involving two upper hydraulic jacks which press on the upper cylinder so to adjust it automatically in lowering and lifting.
Thus by suitable servocontrols of the prior art it is possible to automatically adjust the pass thickness during the rolling phase.
The same need exists presently in the hot rolling of long products (wire-rod, wire, rod, rod sections, etc.).
The existing adjustment art for the rolling of long products is still carried out mechanically by manual screw systems or by electro-controlled reduction gears which require a lot of maintenance.
A related solution using a servo-assisted adjustment stand was proposed by the present invention in another patent application (Italian Application No. IT-UD94A00189). This proposed solution proposes a two-high rolling stand, of the type involving moving orthogonally the respective cylinders removing and approaching them from one another by the sliding of the respective cylinder-holding packings along four tie-rods placed at the respective four angles of the stand, where at least at one end of each of the tie-rods, is applied an hydraulic jack which, fastened to the end of each of the tie-rods, acts by pressure or by retraction against the respective packing which carries the respective rolling cylinder.
This solution has the problem of moving the whole stand and is very complex.
Document SU-A-1205952 discloses a universal stand rolling mill providing the division of the upper cylinder-holding packing into two parts, the upper part being connected to tie rods with threads and comprising a single hydraulic cylinder, being also the lower part of the upper packing connected to the rods via threads.
The purpose of this invention is that of considerably simplifying the self-adjustment system of the distance between the cylinders for the rolling stand, which can be a two-high or a three-high stand, both before and during the rolling so that the cylinders modify their distance according to the variation of the characteristics of the section being rolled.